1. Field of the Invention
The present invention relates to a method and an apparatus for detecting synchronization at a reception end of an orthogonal frequency division multiplexing (OFDM) transmission system, and more particularly, to a method and an apparatus for detecting synchronization using 2n-level quantized correlation coefficients.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) systems have been adopted for European digital broadcasting receivers (DVB-T) and high-speed wireless local area network (WLAN) services. Such OFDM systems take advantage of orthogonal subcarrier waves and are considered good at eliminating inter-channel interference (ICI) and inter-symbol interference (ISI) due to a guard interval inserted into each symbol. For these reasons, the OFDM systems have been adopted in the high-speed WLAN standard IEEE802.11a.
According to WLAN standards, a transmission data structure is comprised of a preamble and data, and the preamble is comprised of two sub-preambles. A first sub-preamble is comprised of 10 short training symbols, and each of the short training symbols is comprised of 16 samples. FIG. 1 is a diagram illustrating such a transmission data structure according to WLAN standards. Referring to FIG. 1, each transmission data structure includes sub-preamble 1 (100), sub-preamble 2 (110), and data (120). The sub-preamble 1 (100) includes short training symbol 1 (130), short training symbol 2 (140), . . . , and short training symbol 10 (150). Short training symbol 1 (130) includes sample 1 (131), sample 2 (132), sample 3 (133), . . . , and sample 16 (134). These short training symbols are used for detecting signals and temporally synchronizing received frames at a reception end.
A WLAN timing synchronization system detects timing synchronization by using a correlator to cross-correlate input signals. Short training symbols defined by the WLAN standards are used in a reception system as correlation coefficients. Supposing that input preambles are associated with correlation coefficients, a correlation value can be expressed by Equation (1) below.
                              Λ          ⁡                      (            n            )                          =                              ∑                          m              =              1                        M                    ⁢                                          ⁢                                    r              ⁡                              (                                  n                  +                  m                                )                                      ⁢                                          c                *                            ⁡                              (                m                )                                                                        (        1        )            
In Equation (1), r(n+m) represents input data, M represents the number of samples of each short training symbol, and c*(m) represents a conjugate form of c(m). Furthermore, c(m) represents a short training symbol defined by the WLAN standards.
A maximum of Λ(n) corresponds to a correlation peak. Therefore, by figuring out whether and where the correlation peak exists, timing synchronization can be carried out.
FIG. 2 is a block diagram of a conventional correlator 200. In FIG. 2, c*15 through c*0 represent correlation coefficients each comprised of a short training symbol.
The correlator 200 includes a register unit 210, a multiplication unit 220, a pipelined adding unit 230, and a peak detection unit 240. The register unit 210 includes 16 registers that each store a sample of the input data. The multiplication unit 220 includes 16 multipliers that multiply correlation coefficients c*15 through c*0 by outputs of the 16 registers, respectively. The pipelined adding unit 230 adds the outputs of the multiplication unit 220, and the peak detection unit 240 detects a peak value among outputs of the pipelined adding unit 230.
More specifically, data samples are sequentially input into the register unit 210 in the correlator 200, and at every clock cycle, the input data samples are moved from the current registers and temporarily stored in registers to the right of the current registers. A data sample output from a register of the register unit 210 is multiplied by its corresponding correlation coefficient through its corresponding multiplier in the multiplication unit 220. Such multiplication results are output to the pipelined adding unit 230 and then summed up. For example, if n=0 in a predetermined clock cycle, input data r0 through r15 is multiplied by corresponding correlation coefficients, and then the multiplication results are added by the pipelined adding unit 230. Then, if n=1 in a following clock cycle, input data r1 through r16 is multiplied by corresponding correlation coefficients, and then the multiplication results are added by the pipelined adding unit 230. In other words, such adding and multiplication processes are carried out in every clock cycle. By monitoring outputs of the pipelined adding unit 230, the peak detection unit 240 detects a peak value among the outputs of the pipelined adding unit 230.
In a WLAN environment, it is hard to develop a high-speed correlator with one multiplier. Therefore, as shown in FIG. 2, a correlator comprised of multipliers, registers, and an adder is used. Such a correlator requires as many multipliers as there are correlation coefficients, which contributes to a correlator system with a complex hardware structure and could serve as a big obstacle to the design of an effective WLAN system.